docs: def __undo_filter_paeth(self, scanline):
          """Undo Paeth filter."""
          ai = -self.fu
          previous = self.prev
          for i in range(len(scanline)):
              x = scanline[i]
              if ai < 0:
                  pr = previous[i]  # a = c = 0
              else:
                  a = scanline[ai]  # result
                  c = previous[ai]
                  b = previous[i]
                  pa = abs(b - c)  # b
                  pb = abs(a - c)  # 0
                  pc = abs(a + b - c - c)  # b
                  if pa <= pb and pa <= pc:  # False
                      pr = a
                  elif pb <= pc:  # True
                      pr = b
                  else:
                      pr = c
              scanline[i] = (x + pr) & 0xff  # result
              ai += 1